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Discussion on SH6255

Here is the explanation from Hywel Wyn Owen & Richard Morgan, Dictionary of the Place-names of Wales, Gomer, 2007 (the most authoritative study of the subject):

LLYN LLYDAW

‘lake of ?furies, llyn, ?llid pl llidiau variant llidau

Llin Lleddau 1536, Lhyn Lhydaw 1722, Llyn Llydaw 1819

An obscure name for a lake enclosed by three of Snowdon’s ridges. Suggestions include lled pl. lledau ‘breadth, width’ and llaid ‘mud’ The plural llidau, although unrecorded, meets most of the problems, and could describe unrelenting, ferocious conditions in certain weather. Whatever its origin, the subsequent development has been influenced by Llydaw ‘Brittany’, an association which prompted some to venture a similar derivation signifying ‘lake dwelling’.